London: At least 13,000 opponents of Syrian President Bashar-al-Assad were secretly hanged in a government jail, where mass executions of upto 50 people at a time occur every week, the Amnesty International said in a report on Tuesday.

A poster of Syrian President Bashar-al-Assad. AP.

The mass hangings took at Saydnaya military prison near Damascus between 2011 and 2015 - and there are clear indications that they are still ongoing, according to the 48-page report, "Human slaughterhouse: Mass hangings and extermination at Saydnaya prison, Syria."

The executions take place after one or two-minute lawyer-less "trials" using "confessions" extracted through torture, the report said.

Harrowing accounts of survivors led the London-based NGO to conclude that the suffering and appalling conditions at Saydnaya have been deliberately inflicted on detainees as a policy of "extermination", the report said.

"On top of these extrajudicial executions, the Syrian authorities are deliberately inflicting brutally inhuman conditions on Saydnaya detainees, with systematic torture, deprivation of food, water, medicine and medical care," it said.

According to Amnesty, the Sadnaya hangings follow a set procedure. Carried out in the middle of night and often twice a week, usually on Mondays and Wednesdays, those whose names are called out are told that they would be transferred to civilian prisons in Syria.

Instead, they are moved to a cell in the basement of the prison and severely beaten over the course of two to three hours (the intensity of the beatings is such that one former detainee described people "screaming like they had lost their minds").

The prisoners are then transported to another prison building (the "White Building") on the grounds of Saydnaya, where they are ultimately hanged in the basement.

Throughout the process, they remain blindfolded. They are informed they have been sentenced to death only minutes before the execution, the NGO added.

After execution, the bodies are secretly buried in mass graves. Their families are given no information about their fate.

Lynn Maalouf, Deputy Director for research at Amnesty International's regional office in Beirut, said: "The horrors depicted in this report reveal a hidden, monstrous campaign, authorised at the highest levels of the Syrian government, aimed at crushing any form of dissent within the Syrian population."

"The cold-blooded killing of thousands of defenceless prisoners, along with the carefully crafted and systematic programmes of psychological and physical torture that are in place inside Saydnaya Prison cannot be allowed to continue."